Control systems



Dec. 15, 1964 E. DURHAM ETAL CONTROL SYSTEMS ll Sheets-Sheet 1 OriginalFiled Nov. 26, 1952 57 L2 TURBINE COND.

TURBINE EGON. 8

m S mm 0 THE nl Nm M 5 4 NN Y 6 G M II D 9 6 F F. m G R m a Y y T B m Iva 6 fwio E. DURHAM ETAL 3,161,180

CONTROL SYSTEMS 1 Filed Nov. 26, 1952 ll Sheets-Sheet 2 Dec. 15, 1964Origma QM i W i 31133 3 3 XQQ INVENTORS AND EDWIN DURHAM By H? E. WEAVERATTORNEY R 2 R a m m m 3 A 11F T 5A..

H 6 4 E 7 H m 2 H m P E P .w Rmmmmwwmwwwwmmmmwwwwwmwmwwwwwmwwmwwwwwwmmwwwwmwwwwwwwmmmmwwmwwmmwwwwwwmwo000000000oooooooooooooooooooooooo00000000000000oooooooooooooooooooooooooooQ I uooooo0ooo000oooooooooooooooooooooooooooooooooooooooooooooooocoooooooooooooooOOOOODOOOOOOOOOOOOOOOOO00000000000OOD000OOOOOOOOOQOOOOOOOOOOOO000000000OOODJJ 0 000000O00O000000OOOOOOOOOOOOOOO0OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOQOOO0Oo o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o oo o o o o o o 0 $3 O 0 o u 4 2 J o o 0 0 2 00000 0 00 0000 0 o o o o o oo o OOOOOOOWOOO0OOOOOOUOOOOOOOOOOOOOOOO FIG. 2

Dec. 15, 1964 D H ETAL 3,161,180

CONTROLSYSTEMS Original Filed Nov. 26, 1952 ll Sheets-Sheet 3 H0OREMOVED BY ATTEMPERATOR SPRAY LL (DESIRED FINAL STEAM TEMP 4 I000 u.I IE g GAS RECIRCULATION E 900 Q o a E CHARACTERISTIC CURVE CONVECTIONSUPERHEATER x E 800 I; g m a; o.

0 2O 40 6O 80 I00 BOILER LOAD (PER OENT) FIG. 3

REOIPcuLATION T RH ANO SH DAMPERS FAN STOPS WIDE OPEN I f I OUNCONTROLLED SH TEMP @7/7 R IOOO-X LL I SA W RA M I I UNCONTROLLED E RHTEMP 3 900 I E P 5 RH DAMPERS OPEN SH DAMPERS OPEN I 800 I I I l I 2 SHDAMPERS THROTTLED RH DAMPERS THROTTLED :5 I I I I I CONTROL POINT LOAO I11"111 IY 500 600 700 800 900 I000 I IOO INVENTORS EDWIN DURHAM HARRY E.WEAVER BY FIG. 4

Dec. 15, 1964 Original Filed Nov. 26, 1952 I I I I I I I I I I l l I I II l l I I l I I l l I I E. DURHAM ETAL $161,180

CONTROL SYSTEMS ll Sheets-Sheet 4 FIG. 5

2M1 AXE A ORNEY RATING RECIRCULATED SUPTR. REI-IEAT STEAM STEAM AIR GASFINAL FINAL PREssuRE FLOW FLOW FLOW TEMP. TEMP.

sF AF RGF T84 TR4 e2 e3 64 e5 58 Ts2 T83 TR3 T84 DIFF J[ 70 MANUALCONTROL STATION I FORWARD 0 O O O I REVERSE g 0 O l STOP o 9 9 o g I 12I REGULATE l I RATE I M M M M M M INN W/W/ FUEL AIR suRTR. REHEAT REcIR.PRIM. REHEATER PRIM. SUPPLY SUPPLY ATTEMPERATOR GAS SUPTR. SUPTR.

VALVES DAMPER PROPORTIONING DAMPERS INVENTORS m EDWIN DURHAM HARRYWEAVER Dec. 15, 1964 DURHAM ETAL CONTROL SYSTEMS 11 Sheets-Sheet 6Original Filed Nov. 26, 1952 wzimDk m4 mwhqwImm mvmh INVENTORS EDWINDURHAM HARRY E. WEAVER g j? 2 110mm 2310 mm zom Dec. 15, 1964 DURHAMETAL CONTROL SYSTEMS ll Sheets-Sheet 11 Original Filed Nov. 26, 1952LOAD STEAM AIR RECIRCULATED MASS FLOW FLOW GAS HEAT FLOW STEAM PRESSURETR4A T848 TR4B TS2A TS3A TS4A TRBA TR4A T828 T835 T848 TR3B TR4B 00al..|n||-|l II M n I 090 u 000 .IITIIIII.I M w n m QQQ n M T s n L oaw I!III M O R T m 08 n C A U N A M RATE RH SH PROPORTIONING S2 RECIR GASFUEL AIR SUPPLY SUPPLY ATTORNEY FIG. I2

tet Gfiice 116L180 Patented Bee. 15, 1864 3,161,180 CONTROL SYSTEMSEdwin Durham, Wadsworth, and Harry E. Weaver, Cleveland, Ohio; saidDurham assignor to The Bahcock 8r Wilcox Company, New York, N.Y., acorporation of New Jersey; and said Weaver assignor to Bailey MeterCompany, a corporation of Delaware Original application Nov. 26, 1952,Ser. No. 322,646, new Patent No. 3,028,844, dated Apr. 10, 1962. Dividedand this application July 7, 1961, Ser. No. 121,780

19 Claims. (Cl. 122-479) Our invention lies in the field of steam powergeneration and particularly in the control of steam temperature inconnection with present day vapor generators. Practically all centralstation capacity presently being installed, or on order, in the UnitedStates, has rated steam conditions above 800 p.s.i.g and 800 FTT; thehigher operating temperature being 1050 FFT at pressures from 1500p.s.i.g. to 2000 p.s.i.g. and rated load from 500,000 to 1,000,000 lb.per hr. The problems involved in the generation and close control of theproperties of steam are quite different now than was the case at thetime of the inventions in this field which are shown in the prior art.In such plants, the upper limit of superheat temperature is governed bythe materials and construction of the turbines served by the steam. Inthe interest of turbine efficiency, the temperature of the steamdelivered to the turbine should be maintained within close optimumlimits throughout a wide range of operation.

With the superheating or resuperheating of the steam in one or moreconvection type heat exchange surfaces, the size and cost of suchsurfaces becomes a material factor in the total cost of the unit and anyimprovement leading to a reduction in the size of superheaters becomesof considerable importance. Usually these surfaces must be made ofespensive high chrome-nickel alloy tubing to satisfactorily handle thetemperatures and pressure encountered.

It is thus a prime desideratum, in the design of such a unit, toproportion the steam generating surfaces and the steam superheatingsurfaces to give a desired final steam temperature at rated load. Atpeak load, in excess of the rated load, the final steam temperature willbe in excess of that desired while at lower ratings the steamtemperature will not equal that desired. This is due to thecharacteristic curve of convection type heat exchangers which have arising function with load (FIG. 3). It is false economy to design thesuperheater for desired final steam temperature at peak load, for at allloads below that value, the unit would produce steam which is below thedesired final temperature. On the other hand, the design of asuperheater to produce the desired final steam temperature at somerating below rated load would require an excessive cost of superheatingsurface and an excessive final steam temperature throughout the upperratings, with consequent danger to the turbine or the necessity ofextracting some of the surplus heat from the superheated steam.

To reach the desired high superheated steam temperature, but not toexceed it, requires careful proportioning of the heat absorbing surfacesboth for generating steam and for superheating it. But even if thedesired temperature be just attained initially by very careful designingat rated load, the temperature will vary during operation by reason ofchanges in cleanliness of the heat absorbing surfaces. Slag will formand adhere to the heat absorbing surfaces in the furnace therebyreducing the effectiveness of such surfaces and raising the furnaceoutlet temperature of the products of combustion. Furnace outlettemperature may also change with percentage of excess air supplied forcombustion, with the characteristics of the fuel burned, and with therate of combustion and the corresponding rate of steam generation. Allof these things will therefore affect the temperature of the gases,whether the superheating elements are located in the furnace where theyabsorb heat by radiation from the burning fuel and products ofcombustion, or whether they are located beyond the furnace where theyabsorb heat by convection only from the products of combustion.

With the furnace volume, as well as the vapor generating furnacesurface, and the vapor superheating surface, fixed and invariable, thepossibility of satisfactorily controlling the final steam temperaturelies in controlling the volume and temperature of the gases contactingthe superheating surfaces. Fuel and air supply must be varied withrating or demand to provide the desired steam flow rate and the desiredsteam pressure. The furnace temperature of the flame and products ofcombustion does not vary greatly with rating. This leaves thecontrollable variables as the mass and temperature of the gassesentering the convection superheating surfaces.

For any given furnace, as load increases, the rate of heat absorbtiondoes not increase as rapidly as the rate of heat liberation; therefore,the furnace leaving temperature will rise. With both the quantity rateand the temperature of the gases leaving the funace increasing withload, it is apparent that a fixed surface convection superheater willreceive a greater heat rate at higher loads than at lower loads and theheat transfer area is usually designed for receiving the volume andtemperature of leaving gases at rated load. Any further increase in theheat release rate supplies to the fixed superheater surface more heat bygas volume and by gas temperature than it is designed for acorresponding excess final steam temperature is experienced. On theother hand, at operation below the rated load, the fixed superheatersurface receives less volume and a lower temperature of gases leavingthe furnace with a corresponding lowering of final steam temperature. Itis therefore, a principal object of our present invention to provide animproved method and apparatus for extracting excess heat from the steamat high rating and for supplying additional heat to the steam at lowratings, to the end that the final steam temperature will approximate auniform value over a range of ratings at each side of the rated loavalue.

We preferably consider a unit which has been designed to provide thedesired final steam temperature at rated load. Throughout an upper rangeof rating between the rated load and a peak load, we apply water sprayatternpcration. As rating decreases, below rated load, we controllablydecrease the percentage of liberated heat which is absorbed by theradiant generating surfaces. At the same time the mass flow of the gasesof combustion is increased, to the end that a greater proportion of theliberated heat is delivered to the convection superheating surfaces thanwould otherwise be the case. This control of the mass flow of combustiongases entering the superheating surfaces is accomplished byrecirculating to the furnace a variable proportion of partially cooledproducts of combustion abstracted from the inlet side of the air heater.The exact location of entrance of the recirculated gases to the furnaceis not a part of the present invention.

A principal object of our invention is to provide an improved method andcontrol system effective in positioning attemperator control valves andin controlling the recirculation rate of gases.

Recirculation of partially cooled products of combustion is nota newdevice. With a water-cooled furnace it is known that theheatavailability of the gases at the entrance'to convection superheatingsurfaces is increased when the percentage recirculated is increased asrating decreases.

This may be 'due to relativewincrease in gas, temperature and/or massflow. The lower rating end of:

the convection characteristic curve is raised While the 7 upper end mayactually be lowered; Thecontrolled change in temperature and/ or massflow rate of theg'asesleaving the furnace may result fromdifferent'eifects .of the introduction of recirculated gases atdifferent furnace locations. One theory that has been advanced is ofdelayed combustion and change in temperature of the combustion process.Another is the blanketing or shielding effect of the recirculated gasesbetween the combustion process and the radiant receiving walls of the.furnace proper. Stillother causes may be the dilutioii'of the v freshproducts of combustion and the heating up. of the contemplated that,under abnormal operating conditions, before maximum overload steamoutput is reached and for a narrow overload range, some degree of sprayattemperatio'n may be carried on in connection with the reheater; .Asthe overload range in the normal loading of a steam plant would bereached infrequently, theeffect'on the overall plant economy will notamount to Theinstallation of a spray attemperator associated with thereheater with provision to automatically bring 3 it into operationshould thetemperature become exces sive at the reheater outlet, isadvantageous'from a safety standpoint in giving full protection againsthigh temperatures which might otherw'isedam'age the steam turbine,

reheater and transmission system.

In the load range below the "control point where gas recirculation isused as a means to-raise the proportion of totalheat remaining in thegases leaving the furnace, we utilize gas flow control dampersassociated with parallel passes containing superhe'ater and 'reheatersurfaces for the purpose of directing an adequate proportion of gasthrough the reheater pass. This method of damper operation in the lowload range below the control point decrease in rating and thus toincrease the heat flow rate ,7

of the gases leaving the furnace and entering the super,-

heating surfaces.

At loads above the control point load, convection superheaters andreheaters naturally tend to ,efiect too high a stearn'temperature, byabsorbing too much heat. By-passing some of the gases aboutthesuperheaten and reheater surfaces'could be effected as a means ofavoiding such excessive heat absorption. This, however, involvesexpensive and spacious structural provisions, and the provision of gasflow control dampers which are subject to operating difficulties. Thistype of corrective measure is also somewhat sluggish in its response tocontrol variables;

Spray attemperation is preferable as a temperature corrective measurebecause of its simplicity in construction,

ease of operation, and low gas pressure drop. It has, however, had thedisadvantage that it has a tendencyto effect a loss in overall steamturbine plant efficiency because of the latentheat loss to thecondenserlof the" is the reverse of that of the high' load range, inthat the reheater damper is open and the proportionate gas flow controlis by the damper associated with the, outlet of the superheater pass.Our present invention has as a primary object the p rovision of methodand apparatus for operating, or

controlling'the operation of, such a vapor generating unit through theutilization ofm'ore advantageous indexes of heat availability to theconvection superheaters and of operation of the unit as a whole.

. In the drawings:

FIG. 1 is a sectional elevation of a vapor. generating and superheatingunit having reheat surface and employing recirculation, attemperationand gas distribution.

, ,FIG'. 2 is a section along theline 2- 2 of FIG, l-in the directionofthe arrows.

FIG. 3 is a graph of characteristics of a convection-type superheater.

vapor resulting, from thereheat'attemperator admixture- 1 without theadvantage of expansion of that vapor through the high pressure turbine.This invention provides for the spray attemperation of steam, withtheminimization,

or avoidance of the use of reheater spray attemperation,

while effecting optimum concurrent control of both the high pressuresuperheated 'steam temperatures and the reheated steam temperatures. V

The present invention provides for the absorption of heat by thesuperheater and by the reheater by placing themin separated controlledgas paths arranged for parallel' fio w. Spray attemperators are arrangedin corinec- 7 tion with both the superheater and the reheater. At a loadwhere the heat carried by the gases going overthev superheatingandreheating surfaces is of such an amount as to result in an excessiveabsorption by' the superheat ing and reheating surfaces, weregulatethegas flow o'ver the reheater sothatit'will absorb just'sufiicient heatto.

bring the final temperature to'the desired value. This. will result in agas. flow over the; superheater surface FIG. 4 is a graph ofcharacteristics ofconvection superheating and reheating'surfaces of aunit similar to that of FIG. 1.

FIG. 5 illustrates a manual control station forregulating the operationof the unit of. FIG. 1.

FIG. 6 diagrammatically illustrates a pneumatic con-v trol systemfor'aunit such as the one of FIGS. 1 and 7 O 2, having a single steam circuitwith spray attemperation in both. the superheating and reheatingportions of the c1rcu 1t, -w1th gas recirculation, and with'gasproportioning over the parallel superheater and reheater paths. 7 V

FIG. 7 illustrates the steam and gas paths of a twin clrcuit vaporgenerating, superheating and reheating vunit employing recirculation,attemperation, and gas distribution over the parallel paths.

FIG. 8 illustrates diagrammaticallya pneumatic control system fora twincircuitvapor generating, super heating 'and'reheatingunit employing gasrecirculation,

attemperation in only the superheating portion of-the twin circuits, andgas distribution between the parallel superheating-and reheatinggaspaths.

which will raisethe superheater absorption suchthjaf if uncontrolleditwill give a delivered stearnftemperature in excess of'theoptimum, butwereducelf this excess by spray attemperation in-the superheatersection;

From an operating standpointit is undesirable to create I too wide anunbalance of gas flow through the 'reheater and superheaterpasses,particularly as an attempt tortrans mit we; mnchfheat to the Isuperheater' metal" may result in its temperature becoming excessive,and it is therefore FIG illustrates a pneumatic control system, somewhat similar to that of FIG. 8, butdiffering as to certain ofthe indexesused as Wellas. the general method and:

PParatus'of automatic control, 2

FIG l0 represents a pneumatic control system for a twin circuit'unitemploying {attemperation in the super heating: portion and with-thepos's'ibility of attemper'ation in the, reheating portion of thecircuit.

P16; 11' illustrates 'a pneuma cf control system some: iftering'therefrorr 'in.

what like that of Fro. 10 but certain respects. j- 1 FIG. 12 illustratesa manual control station for regulating the operation of a twin circuitvapor generating, superheating and reheating unit.

Referring now to FIG. 1 we show therein in diagrammatic form, and notnecessarily to scale, a vapor generating, superheating and reheatingunit in connection with which we will explain our invention. While thisfigure of the drawing shows, for illustrative purposes, only a singlesteam circuit, it will be appreciated that the same general structuralarrangement of FIGS. 1 and 2 may be such that the twin circuit of FIG. 7may be applied. Thus the description pertaining to FIGS. 1 and 2 isapplicable to all of the other figures of the drawing when it beconsidered that such a structural unit may have either a single or atwin steam circuit. FIG. 2 is a section (to a larger scale), in thedirection of the arrows, along the line 22 of FIG. 1. Reference may alsobe had at this time to FIGS. 3 and 4 which show the characteristiccurves for steam superheating and reheating surfaces in a unit of thistype.

The generator is of the radiant type, having a furnace 1 with walls 2 ofvertical, closely spaced plain tubes constituting the vapor generatingportion of the unit. Products of combustion pass upwardly through thefurnace 1 in the direction of the arrow, through a tube screen 3, over asecondary superheating surface 4 and then through primary superheaters 5and 6 and a reheater section 7. A tubular economizer 8 is followed bysets of adjustable dampers 9, 10, 11 (FIG. 2) of which dampers 9 areshown in FIG. 1. Following the dampers 9, 10, 11 the gaseous products ofcombustion may pass to an air heater, but, prior to the air heater,

i.e. after dampers 9, and 11, is the recirculation duct 12 joining therecirculation fan 13 which feeds a distribution duct 14 for a pluralityof entrance ports 15.

Reference to FIG. 2 will show that the secondary superheater 4 isspanned by a primary superheater section 5, a reheater section 7 and aprimary superheater section 6. Thus FIG. 1 shows a sectional elevationthrough the superheater 5 and also shows only the superheater dampers 9.The invention contemplates the desirable sequence of operation ofdampers 9, 10, 11 to controllably vary the fiow of products ofcombustion through the difierent heating sections for distribution ofthe heating gases over the primary superheater and the reheater.

The unit is fired by four horizontal rows of burners which we havedesignated as X, Y, Z and L. There may be one or more burners in eachhorizontal row and the burners are supplied with fuel from a pluralityof mills. The additional secondary air for supporting combustion issupplied to the burner box 16 by a duct 17 under the control of a damper18.

In a unit of the contemplated size the temperature of the heating gasesfirst contacting the secondary superheater 4 will probably be in therange 1800-2200 F. Their temperature may be about 1500 F. at a location20 between the reheater and primary superheater, and about 1000 F. atlocation 21 entering the economizer. Gas temperature at location TG isin a measurable range around 1200 F. In the present description we willdiscuss the use of measured gas temperatures with reference to thelocations 20, TG and 21.

We have diagrammatically shown two forms that the gas temperaturemeasuring instrumentalities may take. In FIG. 2 we indicate that we mayput a temperature averaging sensitive device 22 across the width of theboiler at a location 23, or similarly at the location 20. It is truethat the gases at location 23 have passed over a certain portion of theheating surfaces but a much more practical temperature is obtained herethan at the furnace side of screen tubes 3 or even between the screentubes and the first row tubes of the superheater 4. The temperaturesensitive element 22 may be of the gas filled type or it may be a systemof thermocouples or other devices for averaging the temperature acrossthe path.-

6' In fact, it may consist of a bolotneter system sighted across thepath.

In FIG. 2 We show another arrangement wherein a series of temperaturesensitive elements 24 may be spaced across the path and so connected asto average the temperature values if desired. In FIG. 1 we show one ofthe elements 24 and obviously the vertical location of elements 24 maybe at the temperature measuring 10- cations 20, TG, 21, or otherwise asdesired.

For measuring the gas mass flow (to which reference will later be made)we indicate in FIG. 1 pressure connections 25, 26 leading to a mass flowrate meter designated as A, B, C. Preferably the drop in pressurethrough the sections 5, 7, 6 is sensed and thus we indicate three flowmeasuring devices which are designated respectively at A, B, C and areso indicated on other figures of the drawing wherein these three valuesare used in ascertaining mass heat flow for the three paths.

Following the steam flow path of FIG. 1, it will be noted that steam atsaturation pressure and temperature, from the boiler separation drum,enters the primary superheaters 5, 6 through a header 30 and leaves themthrough a conduit 31 to a spray attemperator 32. Water for the spray ofthe attemperator 32 is admitted through a pipe 33 under control of avalve 34 positionable by a motive means 35, and the rate of supply ofwater (WFI) may be continually measured by an instrumentality 36.Temperature (T51) of the stream reaching the header 30 is measured by aninstrument 38 while temperature (T52) of the steam entering theatternperator 32 is measured by device 39. Steam leaves the attemperator32 through a conduit 40 and its temperature (T33) is measured by adevice 41. The conduit 40 joins a header 42 of the secondary superheater4 and steam leaves the secondary superheater from a header 43, passingthrough a conduit 44 to the high pressure turbine 45. The weight rate offlow (SF) of the steam, the pressure (PS), and the final totaltemperature (T84), are measured in the conduit 44 at the entrance to thehigh pressure turbine.

Steam at relatively low pressure and temperature leaves the highpressure turbine through a conduit 46 joining a spray attemperator 47which is supplied with water through a pipe 48 under the control of avalve 49 positionable by a motive means 50, and the rate of supply (WFZ)of water is continuously measured by a meter 51. Steam leaving theattemperator 47 passes through a con duit 52 to the reheater header 53for the reheater section 7 from which the steam passes to the header 54,through the reheater loop 55, and to the outlet conduit 56 whichsupplies the low pressure turbine 57. Temperature (TR3) of the reheatsteam entering the header 53 is measured by a device 60 while the finalreheated temperature (TR4) is measured by the device 58.

Reference may now be had to FIG. 5 which shows a manual control station61 upon which are located instrumentalities mentioned in connection wtihFIG. 1 as well as the necessary pushbutton stations and rate regulatingrheostats for remotely controlling the regulable devices for manuallyremotely operating the unit in accordance with our invention. On Whatmay be the vertical portion of the control panel we show the steampressure indicator PS, recorders 62 and 63 of steam flow and air flow,recorder 64 of recirculated gas flow, and recorders 65 and 58 of finalsuperheated steam temperature and final reheated steam temperaturerespectively. Either steam flow or air fiow may be used as an indicationof load or rating and it will be understood that the air flowmeasurement in this instance does not include any portion of therecirculated products of combustion. In other words the air flowmeasurement is to be taken, preferably of the fresh air supplied to thefurnace of combustion, at a location which does not include recirculatedgases. The recirculated gas flow (RGF) is continually measuredby a rateof flow meter 64 connected across an orifice 67 in the duct 12 and therate of recirculation is controlled by 7 T a damper 68 positioned by amotive means 69. We further provide indicators 3?, 41, 60 and 66respectively of T52, T83, TR3 and TS4-TR4 difference.

On the bench-board portion of the control station 61 we provideforWard-reverse-stop pushbutton stations 70 for each of the variables tobe positioned remotely. In connection with the fuel supply and airsupply we further provide rate regulating rheostats 71 and 72. It willnow is gradually closed by the vice69.

The flow ofrecirculated gases iscontrolled from indications of ratingand final steam temperature T84. This action takes place from a minimumload of the order of 500,000 lbs. of steam perhr. to the recirculationlimiting 1 control point, as indicated by line TU in FIG. 4. During beevident-that, through the agency of the pushbutton I station 70 and rateregulator 71, the fuel supply means may be regulated to satisfy demand,through the remote this part of the operation, the reheater dampers arewide open, and the primary superheater dampers 9 and 11- are operatedconcurrently with the regulation of recirculated gas damper 68 by thepertinent control elements operation of amotive means diagrammaticallyindicated at 73. Similarly the air supply may be proportioned to thefuel supply through the remote: positioning of motive means 74. Thesuperheater attemperator'valve 34 is positionable by the motive'means 35while the reheater atternperator valve 49 is positionable by the motivemeans and dampers. Selective and/or sequential operation 7 may beobtained as well as proper proportioning fuel and air supply.

FIG. 3 shows the characteristic curve of convection superheating surfacedesigned to providea desired final steam temperature of 1000 F. at ratedload. Through out an upper range of rating beyond the ratedload the theexpected final steam temperature would be above the of the desired 1000P. value and through this range we apply.

Water spray attemperation to remove excess heat from the superheatedsteam. As rating decreases; below. rated load, we controllably decreasethe percentage of liberated heat which is absorbed by the radiantgenerating surfaces by recirculating to the furnace a variableproportionof from a visual indication of thefreheat' final temperatureTR4. When a point corresponding to the reheater design point (say950,000 load) 'is-reached, the superheater dampers 9, 11 are wide openand remain wide open in the load range above or beyond that point.

Above the control point load indicated by line TU, superheated steamisattempera-ted by the sprayattemperator 32 to keep its temperatureatthe predetermined value.

.To accomplish this, the operator observes the final steam temperatureT84 and operates the pushbutton to position the water spray valve 351 toregulate water flow an amount required to limit the final steamtemperature to the desired value. V 7

Through the upper load range (above the reheater design point), thereheater damper 10 is throttled by the operator by manual operationofmeans 76 according to the visual indication of reheat final temperatureTR4. Thus, no

water spray attemperat ion of reheat is required from the control pointto a point W which is a load value of the order of 1,050,000 lbs. ofsteamperhn Above this load, and in the overload range to 1,100,000 lbs.of steam per hr.,- it may be necessaryto use spray water for reheat at-'temperation depending upon the extent towhich it is considereddesirable to limit the heat input into the superheater relative to,reheater when consideringvarious facj tors of which-superheater metaltemperature, steam pres- 'sure drop, and superheater draft loss, may bementioned. 'Whensome such factor limits the load at which all of theattemperation'is superheat attemperation then reheat ,attemperation is'initiated for overload conditions. Al-

though a considerable amount of spraywater has been the partially cooledproducts of combustion.

The graph-of FIG. 4 shows the expected design curve PRfor uncontrolledreheat temperature and M0 for uncontrolled 7 superheat temperature.While these curves are of the same generalshape they diverge withdecrease in rating and the overall design is such that the average pointbetween'the two curves touches the 1000 F. desired final temperaturevalue at frated loa 3 This we term control point load. The controlpoint1oad?'considered with respect to steam temperatures for multiple gaspass units as exemplified, mightbe defined as that 7 load at which thegas flow from the furnace, :when the 1 fuel burning equipment isoperated at optimum efliciency,

has the correct total heat contentflo provide for superheating of thehigh pressure steamfland for reheating of the lowpressure steam to theoptimum predetermined temperatures, there being no operating steps, suchas gas recirculation, taken to modify the amount of heatabsorption'inthe furnace. In a multiple pass unit the gas flowingfromthe furnace isso divided between the passes at the-control point loadoperating ratethatthe optimum temperature of the superheat and reheat isattained. At

loads between this control point load and a predetermined minimum load,the invention involves an'increase in the heat c ontent of the gases formaintaining the final super [heat temperature, and this reference is togases which first pass-over: the secondary superheater and thenover'both sections of the primary superheater. increase infh eat contentis elfecte'dby arecirculating' gas systeniextra'cting heating gas fromnear the entrance to the air heater;

The recirculatedgasdamper 68 is in a wide open posi- I tion" at apredetermined minimum load, and as the load":

increases from that-point to the control point, the damper used forsuperheat ,attemperation between the control point.(the line TU) and theload W, such use for superheated attemperation, has no such detrimentaleffect upon the thermodynamic efficiency of the system as would be theuse of a similar-amount of "spray water for. reheat attemperation. v t

V In the graph of FIG. 4, the line XW, from a load value of 550,000 to1,050,000 lbs. of steam", per hr. represents the control of superheatsteam temperature over that range. The curve M0 represents theuncontrolled superheat temperature which would haveobtained (with heatabsorbing surfaces involved) without, the use of the invention, and theline- PRyrepresents the uncontrolled reheat temperature which would haveobtained without the useof the invention.

Inasmuch as "the pressure andheat content per lb. of

the lowpressure steam returned to the reheater from the loadtolowloadwith' the result that deliverytemperatures from'both thesuperheater'and reheater. will drop and the outlet temperature-loadgraph for the reheater will havea greater slope than thecorrespondinggraph 'ofthe 1 superheater. This is clearly shown by the relative'curves MOand PR. In other'words,,they are of the same general curvedcharacteristiebut'as load decreases they diverge from.each'other..-" I lThe shaded area designated {SA (int nds in l g 7 operation of the motivede- RA) illustrates the extent of the load range through which superheatattemperation is elfected, with the ordinates above line XW indicatingthe increase in amount of such spray attemperation with increase ofload. The smaller shaded area RA illustrates the extent of the overloadrange through which reheat attemperation is eilected and the ordinatesabove the level of line XW indicate the increase in the amount of spraywater so used. It will be noted that area RA is relatively small ascompared to SA and whatever use of spray water in the overload range isnecessary will be of minor importance as regards the overall thermalefliciency of the plant which will usually operate below the 1,050,000lbs. per hr. load.

In the contemplated operation of the unit and apparatus exemplifiedthrough several phases, each involving a different load range, thefollowing takes place with an increase in load from minimum to maximum.

In phase 1, gas recirculation is at its maximum rate and the degree ofthe superheater path throttling is greatest at low load, the reheatergas path being unrestricted. As the load is increased through phase I tothe start of phase II, the gas recirculation is reduced to zero, thethrottling of the superheater gas path is reduced so that when phase IIis entered the superheater is still throttled to some extent, and thereheater pass .is unrestricted. The control point load TU lies betweenphases I and II.

In phase II, the reheater gas pass is continued in an unrestrictedcondition with further reduction in the restriction of gas flow in thesuperheater pass and with concurrent introduction and progressiveincrease in spray attemperation of superheated steam.

There is no restriction to gas flow in the superheater pass in phaseIII, but the reheater gas pass is progressively restricted, andattemperation of the superheater is progressively increased withincreases in load.

In phase IV, the overload range, the superheater gas pass isunrestricted, restriction of the gas flow in the reheater gas pass iscontinued the same as at the termination of phase III, whileattemperation is progressively increased in the superheater andattempcration of reheated steam is initiated and progressivelyincreased. This is effected through operation of the pushbuttons (FIG.by the operator, from indications of final reheat temperature TR4; orautomatically by the sequential and relative operation of the controlinstrumentalities later to be explained.

It will be apparent, from a study of FIG. 4, that during phase (I) therecirculation of gases is regulated, either manually or automatically,to raise the uncontrolled characteristic curves PR and M0 (to the leftof the line TU) until the lower one (PR) coincides with the desiredtemperature line XW. If this were all that were accomplished it wouldmean that reheat temperature were brought to the desired value but thatsuperheat temperature would lie considerably above the desired 1000 P.value. Concurrently with regulation of recirculated gases theproportioning dampers for the superheater and re heater parallel passesare so regulated as to divert some of the total gases away from thesuperheat passes and into the reheat pass. This serves a double functionof taking heat away from the excessively highly superheated steam andadding heat to the reheated steam. By proper adjustment of recirculationrate and proportioning of the reheatsuperheat dampers both the lines PRand MO are made to substantially coincide with the line XW. If, as forloads in phase II in bringing the reheat temperature up to the desiredvalue, it is not feasible to entirely lower the final superheattemperature to the desired values then spray attemperation may bebroughtinto play in the superheat portion of the cycle for extracting theexcess heat therefrom.

As rating increases there is preferably a sequential operation betweenthe recirculation and the attemperation with or without overlap as maybe found necessary. As pointed out there is desirably a sequentialoperation between the reheat pass dampers and the superheat pass 10dampers and this sequential operation may be also sequential relative tothe attemperation, depending upon design and operating conditions.

Referring now specifically to FIG. 6 we show therein an automaticcontrol system for controlling the unit of. FIG. 1 in accordance withour invention. Steam pressure as an index of demand, acting through aBourdon tube, positions the movable element of a pilot valve to controlthe supply of fuel and air to the unit to satisfy demand. The pilotvalve may be of a known type as disclosed in the Johnson Patent2,054,464 and is so arranged that its air loading pressure output iscontinuously representative of steam pressure.

The temperature control for both the superheater and the reheater iswith a minimum of spray attemperation, a minimum of recirculation, and aminimum of draft loss due to the sequential operation of theproportioning dampers. For the superheater the controls are arranged forsequential operation of spray attemperation at high load and flue gasrecirculation for the lower loads. For the reheater the controls arearranged for sequential operation of the spray attemperation, ifrequired at high loads, and the proper positioning of the proportioningdampers at all loads. The reheater obtains the full effect from the fluegas recirculation, the same as the superheater, both in response tochanges of operation and in load range. The reheater and superheater areinterlocked, temperaturewise, to maintain a proper relationship forboiler load outside the control range such as at very low ratings.

FIG. 6 sets the pattern of the invention. While the other drawings showmodification or changes in the control system necessitated because ofthe type of unit there under consideration, basically the invention isas will be described in connection with FIG. 6. We provide fortemperature control at high loads by attemperation of both superheat andreheat steam, and temperature control at lower loads by flue gasrecirculation and the proportioning of gas flow over parallelsuperheater and reheater passes.

For the superheater control the flue gas recirculation is controlledfrom three elements; boiler demand or load represented by total air flowor superheated steam flow, final superheat temperature, and recirculatedgas flow. The spray attemperator operates from the three-elements; finalsuperheat temperature, attemperator outlet temperature and boiler load.The primary load index is used only for superheat control. Thetemperature T83 is useful only when the attemperator is in service (noton recirculatron ratings) because when the attemperator is not inservice then the temperatures TSZ and T53 are the same. Consequentlytemperature T83 is effective only on the attemperator valve.

The rating index AF represents a continuous measure ment of air suppliedfor combustion, through the agency of an air flow rate meter 63connected to be responsive to measured pressure differentials existingacross a restriction at the outlet of the forced draft fan for example.Thus AF represents the air supplied for combustion and does not includeany recirculated products of combustion. The meter 63 is arranged toposition the movable element of a pilot valve 81 thereby establishing inthe pipe 82 a pneumatic fluid pressure continuously representative ofAF.

Final superheated steam temperature is measured by the device 65arranged to position the movable element of a pilot valve 83 and therebyestablish in a pipe 84 a fluid loading pressure continuouslyrepresentative of the value T84.

The pipe 84 joins the A chamber of a standardizing relay 85 which may beof the type described and claimed in the Gorrie Patent Re. 21,804. Sucha relay provides a proportional control with reset characteristics. Itprovides for the final control index T54 a floating control of highsensitivity superimposed upon a positioning conculated gas flow staticpressure.

1 1 trol of relatively low sensitivity. The function of the adjustablebleed connection 86 between the D and C chambers is' to supplement theprimary control of the pressure in pipe 84 with a secondary control ofthe same or of a diflerent magnitude as a follow-up or supplementalaction to prevent overtravel and hunting. The output of the relay 85,available in a pipe 87, is admitted .to the A chamber of a totalizingrelay 88, to the C chamber of which is connected thepipe'82'. Relay 88may be of the type described and claimed in the, patent to Dickey2,098,913 and its outputis available in a pipe 89 leading to amanual-automatic'selector station '90 which is preferably of the typedisclosed in the patent toFitch 2,202,- 485.

The output of the selector station 99 has .ab-ranch pipe 91 joining theA chamber of a relay 92, and a branch pipe 93 joining the A and ,Cchambers of a relay 94. The output of relay 94, through a pipe 95,enters the A chamber of a differential standardizing'relay 96 similar infunction to relay 85. To the B chamber of the sure continuouslyrepresentative of the rateof flow of recirculated gas as measured bydevice 64. The output of relay 96 is available through a pipe 98 andselector station 99 to an accelenatingrelay 100 for control of a fluidpressure responsive motive means arranged to I calibrating relay 104which positions the attemperator valve '34 by way of selector station105 and pipe 106.

It will now be evident that we have described, in connection with FIG. 6the control instiumentalities for the 'loading pressure is subjectedupon the C chamber of relay 92 whose output, available in a pipe 103,joins a recirculation damper 68 and'the attemperator 34 activated 7 afrom four variables in the operation, namely TS4, AF,

RGF and T53, Final superheated steam temperature T84 is the desideratu-mtovbe attained by this portionoi :thecontrol system and provides theprimary control in the A. chamber of relay 88. An anticipating effectfrom the rating index AF provides a secondary control index for therelay 88. The resultant of these effects actsupon both the recirculatingdamper and the attemperator valve.

In this portion'oif the system it will be seen that the demand for waterspray attempenation is established primarily from two factors (T84) andrating (AF). There-' fore, if the actual amount of water sprayed intothe attemperator is correct to accomplish the desired result 12 TU (FIG.4). For further increases in rating, if TS4 tends to increase above thedesired value, primary control from AF and T S3 will position theattemp'er'ator valve 34 approximately the right amount, with a finalvernier adjustment from TS4. V

The damper 68 and the attemperator valve 34am desirably operated insequence, with or withouta slight overlap asmay be demanded by .designor operating conditions. Such sequential operation is readily obtainedthrough adjustability of the calibrating relays 94, 104 which may beadjusted to permit the loading pressure of pipe 89 to position therecirculating damper and not affeet the attemperator valve during thelower ratings.

The selector valve 9% allows for remote manual positioning of the damper68' and of the valve 34, with the sequential interrelation, or bycompletely automatic control of the damperiand valve underthe samesequential conditions of operation, while selector stations 99 and 105allow separate remote manual positioning of the damper and valve. v

The control of reheat final temperature TR4 is accomplishedsimultaneously with the superheat control just described. Primarycontrol from low loads up to the control point load where-recirculationis discontinued, will be from changes in recirculation as determined bythe superheat control. If, howeven'this does not produce the desiredfinal RH temperature. then the proportioning dampers at the exit of SHand RH will be adjusted by TS4-TR4 difference to maintain equality or apredetervmined difference. -'The proportioning dampers are not separatedas to the impulse EZtibut the receiving drives are adjusted forsequential operation. The receiving drives for positioning the dampersets 9-11 and 10 may each be like the power drive shown in FIG. 8 ofGorrie et al. Patent 2,679,829 having a positioner (1) receiving apneumatic signal through the pipe (133) from a relay (134). As disclosedin the Gorrie et a1. patent, the said positioners may be receptive to apneumatic signal in the range 5-25 p.s.i.g. and may be individually soadjusted that the drive which positions the dampers 9-11 willaccomplish, a preselected travel when its positioner is sub- ;ject edtoa loading pressure increasing from 5 to 15 p.s.i.g. while the drivewhich positions the dampers 10 will accomplish its preselected travel asthe pneumatic signal in pipe 120 increases (from 15 to 25 p.s.i.g.; thusaccomplish- .ing a true sequential movement of dampers 9-1 1 and10.

- Qt course the positioning is in reverse sequen'ce the signal pressurein pipe 120 decreases from 25 down to then the temperature T53 of thesuperheated steam leaving the attemperator Will beias desired.

Asrating increases from minimum the AF index will start therecirculation fan and efiect a primary adjustment onfthe gasrecirculation damper of approximately the right amount. A verniercontrol is effected from TS4 through the standardizing relay $5. Thecontrol impulse'of pipe 95 is scanned by the impulse in pipe 97(representative of measured RGF) to provide a readjustment'if necessary..Thisfunctions to insure accurate'positioning of the damper controldrive so as to obtain a definite flue gas flow for each loading pressurein pipe 95 regardless 'ofthe damper characteristic and the recir- Insimilar manner the 5 p.s.i.g., As disclosed in the presentspecification, the

adjustment of the'drive p'ositioners may be such that an overlapping ofmovement of dampers occurs if desired. a For the reheatiportion of thesteam cycle we preferably separate the control of the attemperator valve49 from I coutrolof the proportioning dampersf9, 10, 11.

This because 'a control of the attemperator valve has no react ve efieetupon the final temperature of the superheated steam whereas theproportioning dampers affect the final temperature not only of thereheated steam but also of control impulse in pipe'91 is scanned by animpulse rep resentativeof T83 to obtain a definite water'f fiowrate for'each loading pressure in pipe 91 regardless ofvalve char acteristics,pump pressure, pipe line resistance, etc.

As rating continues to'increase, the flue gas recirculation will befurther and further reduced until the actuator orfdamper 68 reaches a,predetermined minimum position at which point pressureswitches will shutdo'wnthe recircul-ation fan; This occiurs'fa't thecontrolpoin't loadtrol range.

the primary superheated steam. The control of the RH steam temperaturewill the eliectedby operation of the gas flow distribution dampersproportioning the total gases over the parallel paths. As the RH tendsto in- "crease above the control point, the RH damper; will be throttledas required to reducerthe gasflow over RH and divert itto the SH path.On. the 'other'hand, should the RH temperature tend to? go belowstandard, theRH at any time,this being when the other hasreached its,

This true sequence is illustrated limit of etiectiveness. in FIG. 4. Thesequential operation of the proportioning dampers where both are neverthrottled together provides minimum draft loss. anda maximumeffective'con- On the assumption that TR4 and T84 are to be maintainedthe same then any discrepancy or difference between the two is desirablycorrected by way of the proportioning dampers 9, 1i), and 11. Weindicate the temperature difference meter 66 as having what is known asa center-zero and showing by departure therefrom as to whether TS4 isgreater than, or less than, TR4. Correspondingly the air-loadingpressure established in pipe 115 will continually subject upon the Achamber of a relay 116 a fluid loading pressure of predetermined amountif there is no difference between these two final temperatures, but theloading pressure will depart in one direction or the other dependingupon one of the final temperatures becoming greater or less than theother.

The output of relay 116, available in a pipe 117, is subjected through acalibrating relay 118 and selector station 119 to be available in a pipe120 for application to the control drives arranged to separatelyposition the dampers 9, 10, 11. Usually the primary superheater dampers9 and 11 would be operated together and as a group separately from thereheater dampers 10. Under certain conditions, it is desirable tofurther separate and individualize the control of dampers 9 as comparedto dampers 11. In the present embodiment however, we consider dampers 9and 11 as a group but separately regulated in proportion to theregulation accomplished by dampers 10.

We have indicated, in connection with FIGS. 1, 2 and 5, that the motivepositioners for the valves and dampers may preferably be in the natureof electric motors. On the other hand (FIG. 6) the valves 34 and 49 areshown as fluid pressure actuated valves and we state here that themotive means for positioning the dampers groups 9, 1t) and 11 are inthis case fluid pressure responsive motive devices of any of severalwell known forms. Usually these are piston actuated and include thenecessary positioning devices for characterizing and interrelating thepositioning as desired.

As the rating continues to increase, if the gas proportioning is unableto reduce RH temperature sufliciently then the RH attemperator valve isopened under the control of T114 and TR3. Calibrating relays 112 and 118allow adjustability of effectiveness and sensitivity between theproportioning damper control and the attemperator control and also allowsequential operation between the two.

At 119 we show a relay receiving in its A chamber a loading pressurecontinuously representative of TR3 while to the B chamber we apply aloading pressure continuously representative of TR4. The output of therelay, available in the pipe 111 is imposed upon the calibrating relay112 whose output acts through the selector station 113 and a pipe 114 toposition the water valve 49. It will thus be seen that temperature TR3,immediately representative of the effect of spraying water into theattemperator 47, provides an initial index dictating a positioning ofthe valve 49. In order to correct for poor valve characteristics andother deviants the final (desirably uniform) steam temperature TR4 isused as a check-back to readjust, it necessary, the position of thevalve 49.

It will be understood that the final temperature of the superheatedsteam and the final temperature of the reheated steam may desirably bethe same (1000 F7 for example) as is indicated by the line XW of FIG. 4.However, under certain design and operating conditions it may be thatthere is desirably to be a fixed difference between the two finaltemperatures, for example, T54 may preferably be i800 F. while TR4 maypreferably be 950 F. This is readily accomplished by a fixed biasingcalibration so that all pressures and actions of the control system areas though the two temperatures TR4 and TS are to be the same whileactually they are desirably to be maintained at a fixed difference.

The reheat will normally not require attemperation even at high loads asthe reheater is always favored by proper control of gas flow over thereheater with or without the recirculation fan in service.

The parallel arrangement of superheater and reheater with its controlhas a distinct advantage over that of a series arrangement as for loadsbelow the control range of either the recirculation equipment or theproportioning dampers, it will be possible to hold a fixed relationshipbetween the reheater and superheater final temperature. That is, thereheater and superheater temperatures may be made equal in value or heldto a desired ratio or difference for all loads below the control rangeof the equipment. This fixed relationship between the reheater andsuperheat temperatures for low load operation is made possible by theTS4-TR4 difference controller 66 which primarily controls theproportioning dampers. This temperature difference controller isavailable at all loads, but at the higher loads where the temperaturesare maintained at the proper level the temperature differential elementsare relatively inactive. At low loads below the SH controllable ratingwe avoid the possibility of the control trying to maintain RH at theexpense of SH.

Many of the larger units of present day design require dual or twinsteam circuits and we have illustrated diagrammatically in FIG. 7 thefluid circuit of such a unit with a very schematic representation of thegas flow path through the superheaters, reheater, and the recirculatedgas passage. The general principles of our invention which we havedescribed in connection with FIG. 6 apply equally as well to a twincircuit unit with additional features peculiar to the equalization oftemperatures in the multiple circuits as well as for providing safe andefiicient operation.

Steam leaving the separation drum of the unit passes to a headersupplying the primary superheaters 5, 6 which feed into a common header126. Leaving the header 126 the A circuit steam of temperature TSZApasses through an attemperator S1, leaving at temperature TS3A.Similarly steam of the B circuit passes through an attemperator S2 andits outlet temperature is indicated at TS3B. The A and B circuits arecrossed to join a common header 127 supplying the secondary superheater4 which discharges to a header 128. Temperatures TS4A and TS4B designatethe final steam temperatures joining in the conduit 44 to supply thehigh pressure turbine. Steam at relatively low pressur and temperaturedischarged from the high pressure turbine 45 through the conduit 46divides to the reheat circuits A and B which are equipped withattemperators R1 and R2 respectively, and these discharge to the header129 supplying the reheater 7 which then feeds the header 130 and the lowpressure turbine 57.

Gases leaving the primary superheaters 5 and 6 are regulated by thedampers 9 and 11 respectively, while gases leaving the reheater 7 areregulated by the dampers 10. We diagrammatically show the recirculatedgas duct 12 taking a portion of the gases from the several heaters,under the control of damper 68 as previously described.

The unit of FIG. 7 may be remotely manually controlled in accordanceWith the present invention by the arrangement of FIG. 12 to which hasbeen applied the various additional measuring and controllinginstrumentalities for the twin circuits.

FIG. 8 diagrammatically illustrates a pneumatic control system embodyingour invention with a twin circuit steam generating, superheating andreheating unit, utiliz ing gas recirculation, as well as gasdistribution over the parallel superheating and reheating passes. Thedesign characteristics of this unit being such as to not require waterspray attemperation in the reheat circuit, the control of RH temperatureis entirely by the effect of recirculation over the lower ratings andwith proportioning of the gases through the parallel paths. Gasproportioning may be effective for only a portion of the rating range ormay be effective across the control point lead, and

may be adjusted to be operative in sequence with recircu' lation orattemperation,

Basically the indexes used and the control actions etheated steam at theentrance toattemperators S1 and 52 respectively. Any disturbance causedby changes in gas flow, excess air, rating etc'. as well as slagging andother furnace conditions is eifectivc at the primary superheater outletsufiiciently in advance of resulting changes 'or variations in finalsteam temperature as to make the index TSZ avaluable one in controllingthe final temperature T84. I I V We provide at 139 (and at140) acomparing relay to which the threeindcx loading pressures are subjected.The output of relay 139leads-direct-ly to valve S1 while that of relay140 leads directly to valve S2. Calibrating relays 142, 144 provideadjustability for the two valves to take care of valve characteristics,etc.

During the lower ratings, below the control point load, theatternperator valves S1 and S2 are both closed and the control of SHtemperature is by gas recirculation as in FIG. 6. In the interest ofmaintaining the steam temperatures in the A and B circuits" as nearlyalike '(or in desired relation) as is possible we selectively use theloading pressure output of relay 139 or relay 140 in control of therecirculation damper 68. Should we use (for example) the impulse ofpipe'1-41 representative of the A circuit to control the recirculationgas rate the temper-- ature conditions of the B circuit might behigherin which case a-control from A' circuit indexeswould result in anexcessively high B 'circuit tempera'ture. 'Thus' we preferably controlrecirculation selectively from the higher of the two circuitstemperature-wise. tive relay 145, in pipe 146, is the selected signal141, or 143, whichever is the higher. I

As a metering check-back to ascertain if the proper" rate of flow ofrecirculation .gas has been established in ac In this embodiment wepreferably use as an r '16 a heated steam temperature for the B circuit,established as a fluid loading pressure in pipe 143, acts to begin toopen attemperator'valve S2 (upon increasing rating) at approximatelythe'contiol-point load. Thus .the sequential operation primarilyprovides for the use of recirculation at ratings below the control pointload and for attemperation above. However, the attemperators areavailable at any time and should emergency conditions or variationsexist and final temperature tend to exceed the desired value, regardlessof the rating, the. proper attemperator will come into play to reducethe final steam temperature TS4A or TS4B.

Variations in the rate of recirculation will have an elfect upon thefinal reheat steam temperature at the same time as afiecting the finalsuperheat temperature. Secondary or vernier control of the reheattemperature is accomplished by proportioning the totalheating gases as 7between the parallel superheater and reheater passes.

' representative of comparison. .This becomes in effect ananticipating-index useful in correcting for undesirable variations infinal steam temperature TS4 before they ac- As in FIG. 6 theproportioning dampers are positioned responsive to final temperaturedifference responsive device T S4- TR4pfor each of-the twin circuits andthe cir- "cuit showing the greatest divergence between the two Theoutput of 'selecfinal temperatures thereof is the circuit whichdominates the proportioning. This is accomplished through the 'agencyofselective relay 16,4'which selects, for pipe 165, the greatesttemperature difference as shown up in loading pressures of pipes 162 and163.

cordanoe with the demand, and to take care of character- I istics offans, dampers and the like, we further utilize the recirculated gas flowmeter 64 to impress, through the pipe 97, a fluid loading pressure uponrelay-148 in balancing opposition to that coming fromtheselective relay145. Discrepancies in rate of flow of recirculated gas be-' tween thatwhich is desired and the actual rate of flow appear as a loadingpressure in the pipe 150 for position ing the damper 68. v v

In some instances it may be desired to utilize fan power of therecirculated gas fanas an index rather than rate of'fiow of therecirculated gas. Upon large units ofthe type under consideration itmaybe necessary on account of physical locati'omduct work,etc..anddesirable for distribution to split the recirculated gas how and have 4it enterthe furnace at spaced locations: Di'stribution between thetwo-portions is shown 'as being underthe con- *trol of balancing dampers155,156. While damper 68 regulates the total flow of recirculated gases,the total. may, in being split to the two sides, aitect the finaltemsequence control of recirculation, and attemperation, withtemperatures to the same degree. tendency for thesuperheatertemperatures to drift apart,

. V parison type; of, relay.

Signal is led to the reheat damper control drives through a calibratingrelay 1-67'andsimultaneously to the superheat dampers 9, 11 through acalibrating relay 166. Inasmuch as there are two'sets'of superheatdampers 9, 11, the output ot relay. 166 is sent to both damper relays168, 169.

By changing the gas flow balance as between the total reheat dampers andthe total'superheat dampers the superheater outlet temperature TS4A andTS4B may tend to drift apart; Actually, any upset in operation thatwould affect the-reheat temperature should afiect the superheat However,if there is a the super-heater discharge dampers .5, 6 are biased andwill maintain a proper balance. To individualize the superheatjdampers 9and 11, beyond only a comparison with reheat temperatures, we preferablybias the relays 168,

169 responsive to a comparison ofthe final SH tempera I dtur e of thetwo circuits. For this we use a ratio relay 171 receptive-of the fluidloading pressures (pipes 87A,

87B) representative of TS4 A and TS4B.

The resultant loading pressure in pipe 170 is applied oppositely torelays 168, 169. Whileth'elrelay 171 is shown as a ratio relay this. maydesirably be a difference or similar com This 'arrangemenfwill. notalter the.. Sequential operation of the:superheater and reheaterdampers.Furtheror without overlap as may be found necessary to prevent hunting.The adjusta-bility-of the,variousdevices permits that'the three-elementimpulse for the AcirCuit, estab- 1 halted. Similarly thethree-elementrcontrol of supermore, the superheater attemperators are.always standing by just in case the superheatnbutlet temperature oneither .side should climb toohigh'for a short time. Biasing. of thesuperheaterdampers 9:, 11 is imposed on top of thesequence operationbetween the reheater and superheater g as' passes which is impulsedbyreheat temperature;

FIG. 9 illustrates a pneumatic control system for the same general typeof unit described in connection with FIG. 8, namely, a twin steamcircuit unit employing recirculation, with attemperation in thesuperheated steam portion of the circuit, with proportioning of thetotal gas flow over the parallel superheater and reheater passes andwithout attemperation in the reheat portion of the circuit. Herein weemploy a principle disclosed and claimed in the copending application SN283,275, now Patent 3,%40,719, by Paul S. Dickey of basically comparingthe m ss heat flow of the gases contacting the convection heatingsurfaces with the mass heat ilow of the steam being superheated orreheated. The anticipating effect resulting from such comparison is ofparticular benefit in recirculated gas installations and, in certaininstances, supplants the use of a rating index and a recirculated gasflow measuring index. Such a system, including the features of ourinvention is illustrated in FIG. 9 and will now be described.

The fluid loading pressures representative of TSEA and TSdA join a relay13% whose output in a pipe M1 serves to position the atternperator valveSZ-l. Similarly the effects representative of T8313 and are imposed upona relay 14% whose output in pipe 143 serves to position the attemperatorvalve S2.

The recirculated gas damper 68 is under the conjoint control of theoutput in pipe 146, a measurement of steam flow, and a measurement ofmass heat flow rate. These three effects are imposed upon a relay 175whose output, acting through a standardizing relay 176, is available inthe conduit 177 for positioning the damper 63. It is now advisable toexplain the establishment of a fluid loading pressure in pipe 178representative continuously of mass heat flow rate over the convectionheating surfaces.

In connection with HQ. 1 we mentioned the pressure connections 2.5, 2.6at chosen locations in the gas low path over at least a portion of thesuperheating surfaces. We preferably duplicate this measurement for eachof the three passes i.e. the reheater pass 7 and the two superheaterpasses 5 and 6. As diagrammatically indicated in FIG. 1 the pressuretapes 25, 2d are connected. to a measuring device 33d which is arrangedto effectively average or totalize the rate of gas ilow through thethree parallel passes. This total mass flow, compensated for average gastemperature T G, in mass heat flow meter 1% establishes in the pipe 173a fluid loading pressure continuously representative of mass heat flowover the corn vection heating surfaces.

The temperature of the heating gases leaving the unit at dampers h, litand ii, or even at location 21 prior to the economizer 3, does not varymaterially with rating or furnace operation. On the other hand thetemperature of the gases entering the superheating paths as at locations23, Ztl will vary widely both with rate of unit operation as well aswith furnace conditions, heat absorption in the furnace, recirculationof gases, etc. Thus, a considerable span in the travel path of the gasesbetween the furnace and the dampers exists wherein a temperaturemeasurement will reflect, to a lesser degree but still representativethereof, entering temperature variation of the gases and at a measurablylower gas temperature. Preferably we employ a measurement at about thelocation designated T6 in FIG. 1 and would probably employ tempcraturesensitive elements similar to those designated 24. Through this agencywe may ascertain the temperature of the gases contacting thesuperheaters 5, 6 and reheater 7, separately or as an average.

The location T6 for temperature measurement of the gases contacting theconvection heating surfaces will be somewhere along the heat exchangesurfaces where the temperature will reflect furnace temperature andfiring conditions prior to fluctuations in steam temperature within thesuperheating surfaces which would be a result of the changed firingcondition. Thus, the temperatures at TG are chosen as cause indexesrather than making use 18 of result indexes. The primary purpose of thepresent invention is to maintain the final total temperature T84 of thesteam and TR4 of the reheated steam each as nearly constant at theoptimum or desired value as is possible regardless of variations indemand upon the unit as a whole.

As pointed out in greater detail in the Dickey application theconvection heating surfaces are, in effect, a heat exchanger in whichfor every load (SF) we may obtain a value for (Qg.TG), compare values(SF) and (Qg.TG) to see if the heat supply rate is right for the heatdemand rate, control the heat supply rate until it is equal to thedemand rate, and check back from a measure T34 of the final steamtemperature to take care of any discrepancies. Another way of statingthis is that we provide a threeelement control. We control heat inputrate to satisfy heat demand and correct the rate of heat input if thebalance does not result in the desired final heat level temperature) ofthe output.

it will be observed that this is exactly what the controlinstrumentslities of FIG. 9 accomplish. The relay receives a loadingpressure representative of SF as well as a loading pressurerepresentative of mass heat flow. It compares these loading pressure andhas a check-back from the pipe representative of the higher (or lower)of the final steam temperatures TS lA, TS lB through the agency ofselective relay M5. The output of the relay 175, acting through thestandardizing relay 176 is available through a pipe 177 for positioningthe recirculating gas damper 68 to regulate the mass heat flow of thegases passing over the convection heating surfaces.

The control of gas mass heat flow or heat available rate to satisfysteam mass heat flow required (to comensate for the anticipated steamtemperature change with changes in rating and furnace conditions)provides a desirable basis of more directly going to the source ofchanges atfccting final steam temperature (and anticipating the effect)than to depend only on a load index (steam flow rate or air flow rate)with a check-back from final steam temperature.

Control of proportioning of the heating gas flow between the twosuperheater passes 5, 6 and the reheater pass 7, is conjointly from ameasure of mass heat fiow rate and temperature dilference between thefinal superheated stearn temperature T84 and the final reheat ternperature The. The temperature difference meter 66 establishes a loadingpressure in the pipe 115 acting through the standardizing relay 116 andoutput pipe ll? upon the A chamber of a comparing relay which receivesin its C chamber a loading pressure from the pipe 178 representative ofmass heat flow rate. Thus mass heat flow rate, which is effected byrecirculation of partialiy cooled heating gases and is a measure of theavailable heat in the gases, is primarily effective in the reheatcontrol relay 167 and the superheat control relay 166 for proportioningthe gases. The second element in the control is the temperaturedifference between T84 and TR d. As in FIG. 8, the output of relay 166is applied to the relays 168, 169 with the effect of these relays biasedby the loading pressure in pipe 170.

FIG. 10 is somewhat analogous to FIG. 6 except that in FIG. 10 we show atwin circuit with attemperation in both the superheating and reheatingportions although, as previously explained, the entire arrangement issuch as to minimize the amount of atternperation used in the reheatportion of the circuit.

The attemperator valve S1 for the A circuit is under the conjointcontrol of final superheated steam temperature TS4A, steam temperatureTSEA following the attemperator, and a load index AF. These threevariables act through the relay 139 to establish in the pipe 141 a fluidloading pressure for positioning the valve 811. In the same

1. THE METHOD OF OPERATING A VAPOR GENERATING UNIT HAVING DUAL PRIMARYVAPOR CIRCUITS AND DUAL REHEAT VAPOR CIRCUITS FEEDING CONVECTIONSUPERHEATING SURFACES AND CONVECTION REHEATING SURFACES RESPECTIVELY,THE SUPERHEATING AND REHEATING SURFACES DISPOSED RESPECTIVELY IN DIVIDEDAND SEPARATE PARALLEL GAS FLOW PATHS FROM A COMMON COMBUSTION ZONE, THESUPERHEATER AND REHEATER HAVING GENERALLY SIMILAR UNCONTROLLEDLOAD-TEMPREATURE CHARACTERISTIC CURVES WITH DIFFERENT DEGREES OF SLOPE;A METHOD INCLUDING INTRODUCING FUEL AND AIR FOR COMBUSTION INTO THECOMBUSTION ZONE SPACED FROM THE HEATING GAS OUTLET AT A RATE TO SATISFYDEMAND UPON THE UNIT, VARYING THE HEAT AVAILABILITY OF THE HEATING GASESLEAVING THE COMBUSTION ZONE TO THE PARALLEL PATHS TO MAINTAIN SUPERHEATAND REHEAT TEMPERATURES AT PREDETERMINED VALUES THROUGH A LOAD RANGEBELOW A PREDETERMINED CONTROL POINT LOAD IN ACCORDANCE WITH THEPREPONDERANCE OF A PLURALITY OF OPERATIONAL VARIABLES OF ONE OF THE DUALSTREAMS OF PRIMARY VAPOR OVER THE SAME VARIABLES OF THE OTHER STREAM OFPRIMARY VAPOR, AND LIMITING FINAL SUPERHEATED VAPOR TEMPERATURE BYLIQUID SPRAY ATTEMPERATION IN ACCORDANCE WITH AN INDEX OF RATING AS WELLAS SUPERHEAT VAPOR TEMPERATURE LEAVING THE ATTEMPERATOR AND SUPERHEATFINAL VAPOR TEMPERATURE.